A colourless gas X is formed from atmospheric nitrogen and oxygen in a hot engine. In air X is oxidised to a reddish brown gas Y. In sunlight Y splits into X and a reactive atom Z, and Z combines with $\mathrm{O}{}_2$ to give a gas W. W is a harmful pollutant at ground level. Identify W and the change in the oxidation state of nitrogen in the conversion of X to Y.

1. AW = $\mathrm{O}{}_3$; N changes from +4 to +2
2. BW = $\mathrm{O}{}_3$; N changes from +2 to +4
3. CW = $\mathrm{O}{}_3$; N changes from +1 to +4
4. DW = PAN; N changes from +2 to +4